Basically, confocal microscopy consists in a limitation of the field of illumination and of the field of view by means of spatial filtering of the light [1]. The result is the rejection of out-of-focus light and a precise detection of the light coming from a point in the sample.
The main advantages of confocal microscopy in comparison to wide field microscopy are an improved lateral and axial resolution and a higher signal-to-noise ratio for the detection, e.g. allowing fine slice imaging of a specimen under investigation. However, these advantages are at the cost of discrete sampling, i.e. the specimen has to be scanned point-by-point in order to build a full image. Consequently, image formation can be very time-consuming, depending on the aimed image resolution, the size of the imaged area, the method employed for scanning the sample.
Two main categories of scanning modes can be broadly distinguished: stage-scanning and beam-scanning. The second option has become widely used thanks to the combined use of high performance microscope objectives and very fast beam scanning techniques based on Nipkow spinning disks, acousto-optics or rotating mirrors. This combination is widely used for slice-imaging of biological specimens in order to reconstruct 3D images, or for measuring the topography of microelectronic components. However, scanning is only possible within the imaging area of the objective, and a well known trade-off exists between resolution (defined by the numerical aperture of the objective) and field of view. For example, a microscope objective with a high numerical aperture (NA) around 1.2 is restricted to a very limited imaging area, typically well below 1 mm2. Moreover, the objective needs to be finely corrected from off-axis aberrations all over its field of view, and simultaneously corrected for chromatic aberration if polychromatic light is involved. Such corrections require abnormal dispersion materials in the objective fabrication, often leading to a limited wavelength range of use and to a growth of the objective size, complexity and cost.
In contrast, stage-scanning confocal microscopy can be performed using simpler focusing optics, e.g. only operating for a single on-axis conjugation. Moreover, the possibility of reaching wide-area imaging is brought back as the field of view of the employed focusing elements is not a limitation any longer. Nevertheless, stage-scanning of a large surface with a single focus can be very time-consuming because this method is typically slower than beam-scanning.
A wide-field microscopy and micro-projection system that is not relying on any high-NA objective was proposed by K. C. Johnson in 1997 (U.S. Pat. No. 6,133,986). In this device, the wide-field of a low NA projection system is combined with the relatively high NA of an array of microlenses. This scanning confocal system was designed with special attention to microlithography, an application that is far more demanding than conventional microscopy concerning the imaging field-size requirements. Still, the limitations of this system are the low NA of microlenses (NA<0.6) in comparison to microscope objectives, severely restricting its resolution, especially in the axial direction. Furthermore, microlenses are intrinsically suffering from chromatic aberration, which prevents high resolution with polychromatic light.